Transformer core construction and method of producing same



N 1965 w. OLSEN ETAL TRANSFORMER CORE CONSTRUCTION AND METHOD OFPRODUCING SAME 4 Sheets-sheet 1 Original Filed Nov. 13, 1961 INVENTOR.

L L H D W at an D L x wH Y B Nov. 30, 1965 w. OLSEN ETAL 3,220,239

TRANSFORMER CORE CONSTRUCTION AND METHOD OF PRODUCING SAME OriginalFiled Nov. 13. 1961 4 Sheets-Sheet 2 INVNTOR WILLY OL-6EN BY HOWARDu'rmDALL XWM ATTORNEY I Nov. 30, 1965 w. OLSEN ETAL TRANSFORMER CORECONSTRUCTION AND METHOD OF PRODUCING SAME 4 Sheets-Sheet 5 OriginalFiled Nov. 15; 1961 INVENTOR. WILL-Y OLSEN HOVJHRD DPF DALL W ORIVE VNov. 30, 1965 w. OLSEN ETAL TRANSFORMER CORE CONSTRUCTION AND METHOD OFPRODUCING SAME 4 Sheets-Sheet 4 Original Filed NOV. 15, 1961 INVENTOR.

WILLY OLSEN BY HowfiRO D.TIHDHLL ATTORNEY United States Patent C)srs,474

3 Claims. or. 72-394 This invention relates generally to wound typetransformer cores, and more particularly relates to a wound transformercore having superior magnetic characteristics resulting in lower corelosses and higher transformer efficiency, this application being adivision 'of application Serial No. 151,655, filed November 13, 1961.

The superiority of a wound transformer core as compared to cores made ofsolid sections or punched laminations is well-known to workers in theart and need not be detailed herein. Moreover, it is also known that awound transformer core made from a single continuously wound strip ofcore material will normally exhibit better magnetic properties than atransformer core fabricated from a plurality of strips which have endsbutted or lapped to form the composite core. However, as a practicalmatter, manufacturing economics dictate that wound cores should be madefrom a plurality of strips of core material rather than from a singlecontinuous strip even though the joints inherent in such a constructiontend to degrade the magnetic efliciency of the core structure. Thisbeing the case, a great deal of effort has been concentrated on Ways ofminimizing the deleterious effects of the joints while maintaining themanufacturing benefits which flow from the jointed type of coreconstruction.

In nearly all instances it is desirable to form the transformer cores sothat they are of rectangular or square shape and have a correspondingrectangular or square central opening or window to accommodate thetransformer coil structure in order that the overall transformer may bemade as compact as possible, the composite transformer usually includingat least two cores each of which is disposed about one leg of the coilstructure so that one leg of each core is disposed within the coilwindow in such manner as to cause the coil window to be substantiallycompletely filled. This general type of transformer structure createsproblems with regard to the installation of the preformed cores throughthe coil window since there is very little unoccupied coil window spacein the finished assembly and the corners of the core structures must bepassed through the window in order to close the core.

The preformed core corners are necessarily deformed by straightening inorder to be passed through the window, particularly with regard to theouter laminations of the cores, with the consequent introduction ofmechanical stresses and strains into the laminations of the cores. Theintroduced stresses alter the magnetic properties of the core in anadverse manner and are of course undesirable. This condition isaggravated in most wound core constructions by the fact that the corelaminations have a high space factor at the corners and are preventedfrom readily shifting relative to one another as they are stressed whilebeing passed through the coil window. This increases the strainsintroduced at the corners and further degrades the core performance.Attempts have been made in the past to reduce the severity of thisproblem by the use of various forming methods directed toward reducingthe space factor at the corners, as for example by the use of insertshims in the corner regions as the core is being wound. Unfortunately,these known methods are either uneconomic or also tend to reduce thespace factor in the straight sided legs and yoke regions of the corewhere a high space factor is desirable. As will be subsequently seen,the physical realizability of the desirable core structure according tothe invention is related to the method by which the core is made, andthe novel method employed to produce the cores to be hereinafterdescribed is a contributing factor to the superior performance of whichthese cores are capable. Accordingly, it is a primary object of thisinvention to provide an apparatus for producing a wound transformer coreof rectangular or square form characterized by a high space factorthroughout the straight sided regions of the core, and a space factor atthe corners of the core which is sufficiently low that it allows forrelatively free interlaminar movement during assembly of the core to itscoil structure to thereby prevent the creation of mechanical stressesand strains in the core material and preserve the magnetic properties ofan unstressed core.

The foregoing and other objects of the invention will become clear froma reading of the following specification in conjunction with anexamination of the appended drawings, wherein:

FIGURE 1 illustrates a typical wound core according to the inventionshowing the high space factor in the straight core portions and therelatively lower space factor at all of the corners, and alsoillustrating the fact that all of the sections from which the core isbuilt up, excepting the innermost section, are jointed along one of thestraight side legs of the core, the innermost section being jointed onthe opposite core leg;

FIGURE 2 is an enlarged fragmentary showing of the stepped butt jointconstruction of a typical core section;

FIGURE 3 illustrates a pair of core sections built through the window ofa typical coil structure and illustrating the manner in which thesections are built upon one another to form a complete core;

FIGURE 4 illustrates two strips of material bent into surface engagedconcentric circles with their opposite ends respectively abutted,certain geometric relationships being derivable therefrom;

FIGURE 5 illustrates the utilization of the relationships derived fromFIGURE 4 for the purpose of constructing a core strip sizing jig;

FIGURE 6 illustrates in side elevation a stack of laminations previouslycut to their requisite lengths with their righthand ends displaced fromone another preparatory to being bent into a circular loop as one stepin the preparation of the core according to the invention;

FIGURE 7 illustrates in perspective an apparatus for forming arectangular core from a built up core of circular configuration, acircular core being shown in position for forming;

FIGURES 8 through 11 illustrate successive steps in the process oftransforming a sectioned wound core of circular form to a core ofrectangular form in accordance with the novel method according to theinvention; and

FIGURE 12 illustrates the formed rectangular core after removal from thepress apparatus shown in FIG. 5 and ready for annealing.

In the several figures, like elements are denoted by like referencecharacters.

Turning now to a consideration of the drawings, there will be seen inFIGURE 1 a composite core 20 made up of a plurality of internestedindividual core sections, the inner and outer sections being designatedrespectively as 21 and 34 while various of the intermediate sectionshave been designated as 22, 23, and 24. The core is for purposes ofillustration shown as of generally rectangular form having a pair ofside legs 26 and 27 and a pair of yoke portions 28, the straightsidedlegs and yoke portions of the core being joined by the corner regionswhich are so formed that gaps 25 are observed to intervene the adjacentcore sections. In actuality, there also exist slight gaps between theindividual laminations of each core section in the corner regionsthereof although these are not clearly visible because they would tendto confuse rather than clarify the drawing. The inside periphery ofinner core section 21 defines a window 29 within which one leg of thetransformer coil structure is disposed in a completed transformer unit.

It should be observed that while the corners 21a of the innermostlamination of the transformer core are rather sharply defined, all ofthe remaining laminations of the core are of smoothly curvedconfiguration in the corners thereof, the radii of the corner curvatureof the laminations increasing outwardly from the innermost lamination.The curvature of the core section corners combined with the interlaminargaps in the corner regions allow relative movement between adjacentlaminations when the core is being built up section by section upon acoil structure, and prevents mechanical strains from being induced inthe laminations and thereby preserves the magnetic characteristics ofthe core.

By means of a method to be subsequently set forth herein, the loosenessin the corners can be controlled to provide a corner space factor in thepreferred region of 85% to 95% with an optimum space factor obtaining atabout 90%. In contrast to this, wound cores made by the usual methodsheretofore known result in core corners having space factors of 96% to98%, resulting in a corner structure which for all practical purposes isthe same as if the corners were rigidly clamped because such high valuesof space factor prevent corner flexing unless a considerable corneropening force is applied. The necessity for applying such high forces innormal core constructions to effect insertion of the core sectionsthrough the window of the coil structure sets up substantial mechanicalstress and strain in the corner regions which adversely affect the coremagnetic characteristics.

Additionally, it will appreciated that the usual wound corecharacterized by a high corner space factor is not completely strainrelieved by the usual annealing process. This is so because even thoughit is true that a core during the annealing process may be strainrelieved because of longitudinal expansion of the laminations while theyare hot, nevertheless, when the annealed core is cooling down, the outerlaminations are those which cool first and are attended by contractionof these laminations which exert a compressional force upon the not asyet cooled inwardly lying laminations and cause the latter to buckle sothat a wavy condition is produced, in the legs and yoke portions of thecore. This waviness is of course highly undesirable and tends to reducethe space factor in the legs and yoke portions of the core, which areprecisely those portions of the core in which a high space factor isdesired. No such undesired condition occurs in Wound cores madeaccording to the present invention, the space factor achieved in thelegs and yoke portions being very close to 100%.

As best seen in FIGURE 2, the laminations 30 of each of the-individualcore sections, such as for example core section 24, close into abuttingrelationship as at 31, and the ends of the laminations adjacent to oneanother are relatively offset a distance designated in FIGURE 2 as thedistance L, so that a staircase type of composite joint for the coresection is formed. These joints are shown in FIGURE 1 for example as21b, 22b, and 23b for the respective core sections 21, 22, and 23.Additionally it will be observed that the core section joints 22b and231) are longitudinally offset within the core side leg 27 and that thisrelationship holds for the remainder of the core section jointsextending outward through the core side leg 27 to the outer section 34.Thus, the offset joints of the several internested core sections aredistributed lengthwise of the core leg 27 to minimize the magneticreluctance across a plane extending perpendicularly transversely throughthe leg 27 with the core overall joint length being greater than thethickness of the core leg 27 in which the joint is located.Significantly, it will be observed that the joint 21b for the coresection 21 does not lie in the leg 27 but is disposed within the coreleg 26. The significance of this feature is most clearly seen from theshowing of FIGURE 3 to which attention should be now directed.

In FIGURE 3 there will be seen a transformer coil structure having apair of legs 32 and 33, the leg 32 being disposed within the window ofthe lefthand core structure 20' and the leg 33 being disposed within theWindow of a second righthand core structure 20 shown in partly assembledcondition. It will be observed that the WiIldOW 35 of the coil structureis substantially filled by the completely assembled side leg 27 of thelefthandcore structure 20' and the assembled sections of the leg 27 ofthe righthand core 20', there remaining sufficient space for insertionof the ends 36 of the outer core section 34' of the righthand core. Itwill be appreciated that by assembling the individual core sections sothat the section gaps are disposed within the window 35 of the coilstructure, it is only necessary to feed straight portions of the coresection into the window.

Stated somewhat differently, excepting for the innermost section, it isnot necessary to pass any of the core section corners through the coilstructure window. Obviously, in the illustrated case of FIGURE 3 no coresection corner could pass through the window without being practicallycompletely straightened out, the necessity of doing which would causethe severe mechanical strains and stresses in the core material cornerspreviously discussed. Nevertheless, this is exactly what is done in theassembling of most known types of core structures because these coresare almost always constructed so as to be openable through the core yokeregion or through the core leg corresponding to those designated as 26.The difiiculty of passing a core section corner through the coil windowbecomes increasingly more acute as the window size is reduced due to thebuildup of. the core structure.

It will be observed however in both the showings of FIGURE 1 and FIGURE3 that the innermost core sections 21 and 21' respectively each has agap region disposed in the leg opposite to that in which the gap regionsof the other core sections are placed. The reason for this is that sincethe inner core section is in effect the form on which the remainder ofthe core sections are placed and determines the initial size of the corewindow, it is almost essential that there should be a guarantee that thegap of the inner core section be completely and properly closed. Byplacing the gap of the inner section on the outside of the coil leg, andnot in the window, such proper closing of the gap can b assured, sincethe joint is clearly visible and easily manipulable when placed in thislocation. Further, this orientation of the inner core section joint doesnot pose a real problem with regard to the installation of the innercore section about the coil form because the coil structure window iscompletely open at this time and the coil section corners may be rathereasily passed therethrough without any material flexing or straighteningthereof, so that the mechanical stress situation, which becomesprogressively worse as the cores are built up, does not really exist forthe innermost core section.

While FIGURE 3 shows the typical core sections as being each composed offour laminations, this is by no means a necessary condition orlimitation and the individual core sections may be formed with as manylaminations as are deemed desirable. In this regard, FIG- URE 2illustrates the core section 24 as being built up of 15 laminations.Moreover, it is not necessary that all of the core sections of any givencomposite core should contain the same number of laminations.

Understanding now the novel structure of the wound transformer coreaccording to the invention, the method of assembling such a core into anassociated coil structure and the improved performance of which the coreis capable, attention should now be directed to the remainder of thefigures for an understanding of the method of making the novel corestructure and the apparatus employed for carrying out the method.

Each of the rectangular core sections which together make up thecomposite core shown in FIGURE 1 is made from a plurality of pre-cutstraight strips of core material of graduated length produced by anydesired suitable means. The strips are stacked and bent into a circlewith the shortest strip occupying the inside position in the loop andwith the longest strip occupying the outside position in the closedloop. The loop sections are then placed in a press and the circularcrosssection of the core is then formed into a rectangular section. Theformed core is then annealed and disassembled section by section forsubsequent installation onto a coil structure.

Referring to FIGURE 11 there will be seen a pair of strips S and S eachof thickness T and folded into internested circular form with the endsof each strip brought into abutment. The strip S has a mean diameter Dand a mean circumference C while the strip S has a mean diameter D andmean circumference C The strips S and S of course correspond to any twoadjacent laminations of the core to be constructed, these laminationsbeing in circular form in the configuration assumed prior to rectangularforming. It is apparent that the strip S must be longer than the strip Sin order that its ends shall be able to close, and it is necessary todetermine what the difference in strip length should be so thatsuccessive strips of properly increasing length may be cut to form thecore. The difference in length between the strips S and S is thedifference between their mean circumferences C and C The meancircumference of S is,

C =7rD whereas the mean circumference is S is,

C =1rD +27rT Therefore,

C C =AC=21rT From this last relationship it is observed that thedifference in length AC between any two laminations is a function of thelamination thickness T.

In FIGURE 5 it will be observed that the horizontal .line 45 and theupwardly inclined line 46 define therebetween an acute angle 0 Alsoshown are a pair of rectangles 47 and 47a each of which is of a heightdesignated as T. The lefthand end of the rectangle 47a is positioned tothe right of the lefthand end of the rectangle 47 by an amount equal to21rT or 21r times the height of either of the rectangles. The inclinedline 46 is pivoted about the apex 48 of the angle 0 until it is tangentto the upper lefthand corners of the rectangles 47 and 47a, asillustrated at 49 and 50. Thus,

It should be noted that the tangent function of 0 is a constant, andhence the angle a is completely independ ent of the height T of therectangles 47 and 47a. By considering the rectangles 47 and 47a ofFIGURE 12 to be the ends of any two core lamination strips each ofthickness T, it will be noted from the relationship developed fromFIGURE 4 that the difference in length between successively cut stripsshould be 217T to obtain a proper closure of the joints. However, sincethe critical angle 0 defines the angle which theoretically results inexact abutment of opposite ends of each lamination of a stack bent intocircular form together with the maintenance of complete surface tosurface contact between adjacent laminations, it will be appreciatedthat when the circular stack is rectangularly formed there would exist aspace factor at all points of the core cross-section. Since the novelcore construction according to the invention is characterized bylooseness in the corners, it is necessary that the difference in lengthbetween successively cut strips should be slightly longer than thatwhich would result from the utilization of the critical angle 0 This isachieved by reducing the actual angle below the critical angle, anactual angle of approximately 8 having been found to be satisfactory.

By so reducing the angle 0 to below its critical value, there isobtained a degree of looseness between successive strips in thelaminated core structure sufiicient to render differences in length ofthe successive strips inconsequential due to variations in thicknessthereof. As will be apparent, the essential requirement is that eachlamination strip forming a turn of the core be properly butt-jointed andby providing suflicient looseness between successive turns, minorvariations in the lengths of the strips due to thickness variations willnot prevent proper butt-jointing of their ends but instead will resultonly in varying in minor degree the looseness between certain successiveturns of the core structure.

When the strips for a given core section or core have been cut torequisite length in accordance with the foregoing principles, they areflat stacked in the manner shown in FIGURE 6. This particulararrangement of stacking is effected by end shifting the strips relativeto one another so that the ends of the adjacent strips are offset by anamount designated as L in FIGURE 6. This of course causes the oppositeends of these strips to be relatively offset by an amount equal toAC-I-L. The end offset distance L is a function of the thickness T ofthe core strip and is chosen to be large relative to the strip thicknessin order to minimize the reluctance introduced by the joint created byabutment of opposite ends of each strip of core material. The L/ T ratiois normally chosen to fall between a lower limit of 6 to 1 and an upperlimit of 10 to 1, a ratio of 8 to 1 being completely satisfactory formost purposes.

With the stack of strip material positioned as illustrated in FIGURE 6,clamping pressure is applied transversely, as for example at the pointsindicated by the arrows AA, to hold the stack together while it is bentinto circular form to thereby form a circular section, as for exampledesignated by any one of the sections 68 through 71 shown in FIGURE 8.As shown in FIGURE 8 it is also to be understood that a plurality ofsections may be stacked and simultaneously circularly formed as forexample the sections 69, 70 and 71, or alternatively the sections may beindividually formed and then internested to provide the composite arrayshown in FIGURE 8 and including the section 68. It should be noted thatthe joints of the sections 69, 70 and 71 are located in a manner shownfor the joint of the side leg 27 of the core shown in FIGURE 1, and thatthe joint of the section 68 lies in the opposite leg. Further, theorientation of the central forming mandrel 72 relative to the joints ofthe core sections 68 through 71 should be noted. The significance ofthis orientation will appear shortly hereinafter when the operation ofthe forming apparatus shown in FIGURE 7 is understood.

The forming apparatus of FIGURE 7 includes a flat bed or table 73 uponwhich are fixedly mounted a stop block 74 and three hydraulicallyactuated piston cylinders 75, 76 and 77. The cylinders 76 and 77 arespaced apart with their longitudinal axes in alignment, and the cylinder75 and stop block structure 74 are similarly aligned along an axisperpendicular to the alignment axis of the cylinders 76 and 77. Thepiston of each of the cylinders is actuated by a hydraulic pressuresystem (not shown) coupled thereto through the fittings 78 and hoses 79.The actuatable pistons which are reciprocable within the cylinders 75,76 and 77 have affixed to their outer ends rams 75a, 76a and 77arespectively, only the ram 77a being visible in the showing of FIGURE 7.Disposed flatwise against each of the rams is a forming plate 80, 81 or82, a similar forming plate 83 being held fixed position by the stopblock 74. The forming plates 82 and 83 are employed to elongate thecircular core structure with the length of these plates defining theultimate overall length of the rectangular core to be formed. Theforming plates 80 and 81 are the end forming plates and transform thecore from an elongated oval into the final rectangular form. The coreshape transformation is effected by the apparatus in FIGURE 7 in thefollowing manner.

A circular core stack 84, such as the one shown in plan view in FIGURE 8is placed on the table 73 between the forming plates, and a centralforming mandrel 72 is placed in the window 85 of the core stack, as alsoshown in FIG- URE 8. The core stack is prevented from spontaneouslyunwinding by means of the tape strips 88. With the core stack .84 sodisposed, the piston of the cylinder 75 is actuated to drive the ram 75atoward the stop block 74 and thus to move the forming plate 82 towardthe forming plate 83. As the ram 75 moves the forming plate 82 towardthe forming plate 83, the core stack 84 begins to elongate in the mannershown in FIGURE 9. It should be noted that the corners of the centralforming mandrel 72 do not impose any stresses or strains upon thoseportions of the laminations which form the corners of the core or coresections. The movement of the ram 75a continues until the conditions ofFIGURE 10 obtain wherein it will be seen that the core stack 84 is nowtightly compressed between the forming plates 82 and 83 on the outsideand the central forming mandrel 72 in the window region. Thesignificance of the orientation of the central mandrel 72 is nowobserved to reside in the fact that all of the joints of the core stack84 are clamped and there exists no possibility of overriding of the endsof adjacent laminations or the opening of the joints when the formingplates 80 incl 81 begin to move inward.

The tight clamping effected by the forming plates 82 and 83 togetherwith the forming mandrel 72 causes all of the excess length of materialin each of the core strips to run outward toward the ends, and when theforming plates 80 and 81 are moved inward as shown in FIGURE 11, theclamping pressure exerted on the now defined yoke portions, or shortlegs, of the formed core causes the excess material to be forced outinto the corner regions to thereby lower the space factor at thecorners. It now merely remains to apply the securing bands 86 and clamps87 to the outer periphery of the forming plates, retract the rams 75a,76a and 77a and remove the banded formed transformer structure from theforming apparatus. After annealing, the banding clamps 87 are removedand the annealed core may be released from the forming plates and thecentral mandrel 72 removed to reveal a finished core structure asillustrated in the showing of FIGURE 1.

While the present invention has been described in connection with theproduction of cores formed of a plurality of internested single turnlaminations, it is to be understood that the forming apparatus shown inFIGURE 7 and the forming method as carried out in FIGURES 8 through 11may be utilized in the making of complete cores or core sections woundfrom a single continuous strip of core material or from a plurality ofstrips of core material each of which is formed into as many turns asthe strip length will permit. Further, it will be understood that theterm wound as employed in the foregoing specification and in theappended claims is intended to cover cores and/ or core sections inwhich the laminations thereof are formed of separate lengths of stripmaterial successively arranged in concentric relation or of a continuouslength thereof and wherein the strip material is bent lengthwise so thatthe plane of its surface is always parallel to the axis of the core.

Having now described our invention in connection with a particularlyillustrated embodiment of a core structure and the method and apparatusutilized in producing the same, it will be appreciated thatmodifications of our invention may now occur from time to time to thosepersons normally skilled in the art without departing from the essentialscope or spirit of the invention, and accordingly it is intended toclaim the same broadly as well as specifically as indicated by theappended claims.

What is claimed as new and useful is:

1. Apparatus for forming a substantially rectangular transformer corehaving a substantially rectangular central window from a non-rectangularwound transformer core built up of layers of magnetic core stripmaterial and having a non-rectangular central window, comprising incombination, a forming mandrel having a pair of parallel facespositioned in fixed relation to one another and being spaced apart adistance equal to the shorter dimension of the rectangular window to beformed, each of the parallel faces of said forming mandrel being of alength shorter than the longer dimension of the rectangular Window to beformed and sufficiently short so that the mandrel may be disposed withinthe non-rectangular window of the non-rectangular core withoutmaterially distorting the latter, a first pair of spaced apart formingplatens having parallel faces presenting toward one another and eachbeing of a length equal to the longer dimension of the rectangular coreto be formed, said forming mandrel being disposable between said firstpair of platens so that one of the parallel faces of the mandrelpresents toward and is parallel to the face of one of the formingplatens and the other parallel face of the mandrel presents toward andis parallel to the face of the other forming platen, a second pair ofspaced apart forming platens having parallel faces presenting toward oneanother and each being of a length greater than the shorter dimension ofthe rectangular core to be formed, the parallel faces of said secondpair of platens being oriented perpendicularly to the parallel faces ofsaid first pair of platens and being disposed respectively laterallyoutward beyond the opposite ends of said first pair of platens, firstmeans for relatively moving said first pair of platens toward oneanother while maintaining the parallel relationship of the platen faces,and second means for relatively moving said second pair of platenstoward one another while maintaining the parallel relationship of theplaten faces to cause the faces of the second pair of platens toeventually abut the opposite ends of said first pair of platens, theface of one of the second pair of platens abutting one end of each ofsaid first pair of platens and the face of the other one of the secondpair of platens abutting the opposite end of each of said first pair ofplatens, whereby, when a core to be formed is placed between said firstand second pairs of platens and said mandrel is disposed within thewindow thereof and oriented with respect to the faces of said first pairof platens as aforesaid, said first pair of platens may be relativelymoved toward one another by said first means to compress the corematerial between the faces of said first pair of platens and theparallel faces of said forming mandrel to tightly compact the layers ofmaterial which form the longer legs of the rectangular core, and saidsecond pair of platens may be relatively moved toward one another bysaid second means until the faces thereof abut the ends of said firstpair of platens as .aforesaid to compress and substantially square offthe core material at the ends-to form the shorter legs of therectangular core.

2. The apparatus according to claim 1 wherein the length of each of saidsecond pair of forming platens is not only greater than the shorterdimension of the rectangular core to be formed but additionally does notextend laterally outwardly beyond the ends of the said first pair ofplatens when the latter have been moved toward one another to theirlimit when compressing a core disposed therebetween with the saidforming mandrel in the WI? W n w, whereby, after core forming has beencompleted, said platens forrn aim-sided closed rectangular ReferencesCiteii 2y the Examiner may be peripherally secured by means of band-UNITED STATES PATENTS 3. The apparatus according to claim 1 wherein said1,237,015 8/1917 first and second means for relatively moving said first5 1495959 5/1924 Mavlty 153*16 and second pairs of platens respectivelytoward one an gg sfig other comprise reciprocable rams which engage thenonlfigning rear sides of at least one of the platens of each CHARLES W.LANHAM, Primary Examiner.

1. APPARATUS FOR FORMING A SUBSTANTIALLY RECTANGULAR TRANSFORMER COREHAVING A SUBSTANTIALLY RECTANGULAR TRAL WINDOW FROM A NON-RECTANGULARWOUND TRANSFORMER CORE BUILT UP OF LAYES OF MAGNETIC CORE STRIP MATERIALAND HAVING A NON-RECTANGULAR CENTRAL WINDOW, COMPRISING IN COMBINATION,A FORMING MANDREL HAVING A PAIR OF PARALLEL FACES POSITIONED IN FIXEDRELATION TO ONE ANOTHER AND BEING SPACED APART A DISTANCE EQUAL TO THESHORTER DIMENSION OF THE RECTANGULAR WINDOW TO BE FORMED, EACH OF THEPARALLEL FACES OF SAID FORMING MANDREL BEING OF A LENGTH SHORTER THANTHE LONGER DIMENSION OF THE RECTANGULAR WINDOW TO BE FORMED ANDSUFFICIENTLY SHORT SO THAT THE MANDREL MAY BE DISPOSED WITHIN THENON-RECTANGULAR WINDOW OF THE NON-RECTANGULAR CORE WITHOUT MATERIALLYDISTORTING THE LATTER, A FIRST PAIR OF SPACED APART FORMING PLATENSHAVING PARALLEL FACES PRESENTING TOWARD ONE ANOTHER AND EACH BEING OF ALENGTH EQUAL TO THE LONGER DIMENSION OF THE RECTANGULAR CORE TO BEFORMED, SAID FORMING MANDREL BEING DISPOSABLE BETWEEN SAID FIRST PAIR OFPLATENS SO THAT ONE OF THE PARALLEL FACES OF THE MANDREL PRESENTS TOWARDAND IS PARALLEL TO THE FACE OF ONE OF THE FORMING PLATENS AND THE OTHERPARALLEL FACE OF THE MANDREL PRESENTS TOWARD AND IS PARALLEL TO THE FACEOF THE OTHER FORMING PLATEN, A SECOND PAIR OF SPACED APART FORMINGPLATENS HAVING PARALLEL FACES PRESENTING TOWARD ONE ANOTHER AND EACHBEING OF A LENGTH GREATER THAN THE SHORTER DIMENSION OF THE RECTANGULARCORE TO BE FORMED, THE PARALLEL FACES OF SAID SECOND PAIR OF PLATENSBEING ORIENTED PERPENDICULARLY TO THE PARALLE FACES OF SAID FIRST PAIROF PLATENS AND BEING DISPOSED RESPECTIVELY LATERALLY OUTWARD BEYOND THEOPPOSITE ENDS OF SAID FIRST PAIR OF PLATENS, FIRST MEANS FOR RELATIVELYMOVING SAID FIRST PAIR OF PLATENS TOWARD ONE ANOTHER WHILE MAINTAININGTHE PARALLEL RELATIONSHIP TO THE PLATEN FACES, AND SECOND MEANS FORRELATIVELY MOVING SAID SECOND PAIR OF PLATENS TOWARD ONE ANOTHER WHILEMAINTAINING THE PARALLEL RELATIONSHIP OF THE PLATEN FACES TO CAUSE THEFACES OF THE SECOND PAIR OF PLATENS TO EVENTUALLY ABUT THE OPPOSITE ENDSOF SAID FIRST PAIR OF PLATENS, THE FACE OF ONE OF THE SECOND PAIR OFPLATENS ABUTTING ONE END OF EACH OF SAID FIRST PAIR OF PLATENS AND THEFACE OF THE OTHER ONE OF THE SECOND PAIR OF PLATENS ABUTTING THEOPPOSITE END OF EACH OF SAID FIRST PAIR OF PLATENS, WHEREBY, WHEN A CORETO BE FORMED IS PLACED BETWEEN SAID FIRST AND SECOND PAIRS OF PLATENSAND SAID MANDREL IS DISPOSED WITHIN THE WINDOW THEREOF AND ORIENTED WITHRESPECT TO THE FACES OF SAID FIRST PAIR OF PLATENS AS AFORESAID, SAIDFIRST PAIR OF PLATENS MAY BE RELATIVELY MOVE TOWARD ONE ANOTHER BY SAIDFIRST MEANS TO COMPRESS THE CORE MATERIAL BETWEEN THE FACES OF SAIDFIRST PAIR OF PLATENS AND THE PARALLEL FACES OF SAID FORMING MANDREL TOTIGHTLY COMPACT THE LAYERS OF MATERIAL WHICH FORM THE LONGER LEGS OF THERECTANGULAR CORE, AND SAID SECOND PAIR OF PLATENS MAY BE RELATIVELYMOVED TOWARD ONE ANOTHER BY SAID SECOND MEAN UNTIL THE FACES THEREOFABUT THE ENDS OF SAID FIRST PAIR OF PLATENS AS AFORESAID TO COMPRESS ANDSUBSTANTIALLY SQUARE OFF THE CORE MATERIAL AT THE ENDS TOI FORM THESHORTER LEGS OF THE RECTANGULAR CORE.